356 
MAGNETIC SURVEY OF A PART OF THE SOUTHERN HEMISPHERE. 
for we have 
Cape . . —■0I4]2 = C'x 1-363 + P'x 1-669; also+-00742=-l-363F'-l-669Q' 
Mauritius . -*01550=C , X P373+P'X 1*467; also +-00514= - P373F'- P467Q' 
Kins - George’s] 
> — *03295 = C' X 2- 15 1 -fP' X 1*393; also +"00658= — 2*15lF / — 1 -393Q'. 
Sound . J 
Hence by elimination we obtain 
C'= — -0209; F' = — ’0006 ; 
P' = +-0088; Q'= — *0034. 
££ From the values of C', P', F' and Q f , a table of the values of B and C in different 
dips and intensities was formed, and from them with the mean values of A, D and 
E, a table for correcting the observations of Declination was calculated by equ. 35. 
The corrections thus obtained appear to give very closely the true corrections, at all 
events much within the limits of observation errors. The following is a comparison 
between the observed and calculated deviations at King George’s Sound, 6 being 
= -65^04', and <p= 170. 
Ship’s head. 
2 by calcula- 
tion. 
S by observa- 
tion. 
Difference. 
Ship’s bead. 
1 by calcula- 
tion. 
$ by observa- 
tion. 
Difference. 
N. 
-}-6 52 
O / 
+ 0 15 
-0 37 
s. 
+ 0 03 
+ 0 50 
4-0 47 
N.N.W. 
+0 17 
0 00 
—0 17 
S.S.E. 
+ 0 47 
+ 0 55 
4-0 08 
N.W. 
-0 21 
+ 0 20 
+ 0 41 
S.E. 
+ 1 25 
+ 2 20 
4-0 55 
W.N.W. 
-0 58 
— 1 40 
— 0 42 
E.S.E. 
+ 2 02 
+ 3 10 
4-1 08 
W. 
-1 25 
-1 40 
— 0 15 
E. 
+ 2 25 
+ 2 40 
4-0 15 
W.S.W. 
— 1 16 
-1 50 
-0 34 
E.N.E. 
+ 2 17 
+ 3 10 
4-0 53 
s.w. 
-0 54 
— 1 00 
-0 06 
N.E. 
+ 1 52 
+ 3 30 
4-1 38 
s.s.w. 
-0 24 
— 0 24 
+ 0 09 
N.N.E. 
+ 1 22 
4-2 35 
+ 1 13 
+ Sign denotes a deviation towards the west. 
££ It appears from this comparison, that the calculated corrections are smaller in 
amount than the observed. As the ship had just returned from a high magnetic lati- 
tude, it is probable that the observed corrections belonged to a greater dip than the 
one at the station, and therefore that the corrections would be more nearly repre- 
sented by taking them out from the Table for a larger Inclination and Intensity. 
The great differences on the E.S.E., N.E. and N.N.E. points are caused most pro- 
bably by errors of observation. 
££ The correctness of equation (6.) will be more easily perceived by the accordance 
of observations made at sea, in a high dip, making due allowance for the difficulty 
of observing in bad weather. 
££ 2. Calculation of Corrections for the Inclination Observations. 
££ To obtain these corrections four constants are necessary, viz. a,b, c, d; a and b are 
obtained from the deviations of a compass (placed on the same spot as the dipping- 
